Timer control circuits for automatically controlling a cycle of operation of a laundry dryer have been utilized for almost 50 years. There has been continuing efforts over that period of time to make such control circuits simpler, more reliable and less expensive, all to the benefit of the consumer. One problem arises in such control circuits in that during an automatic drying cycle, an exhaust air thermostat is used to operate both a cycle timer motor and a dryer heater. However, normally the dryer heater operates at 240 volts ("v"), whereas the cycle timer motor operates at 120 v. One solution is to use a two pole thermostat so that the cycle timer motor and heater can be simultaneous but independently switched with two different sets of contacts operated by the thermostat. Such two pole thermostats are relatively expensive, and therefore, many dryers employ a simpler circuit as illustrated in U.S. Pat. Nos. 4,868,997; 4,642,907; 4,132,008 and 3,942,265.
Referring to FIG. 1, of U.S. Pat. No. 4,868,997, during the automatic drying cycle, timer switch 22 is open; and the timer motor 16 is connected to a dropping resistor 25 which in turn is connected to a heater 11. At the beginning of the cycle, the contacts of thermostat 12 are closed, thereby connecting the heater 11 to power line L1. Opening the contacts of thermostat 12 removes the shunt from around the timer motor 16; and current then flows through the timer motor 16, the resistor 25 and the heater 11 between the 240 volt power terminals L1, L2. The resistor 11 and the resistance of the timer motor are sized so that approximately 120 v is dropped across the resistor 11, and 120 v is applied to the timer motor 16, thereby turning the timer motor 16 ON. The resistance of the heater 11 is very small and therefore only a minimal voltage drop exists across the heater 11; and current flow through the circuit comprised of the timer motor 16, the resistor 25 and the heater 11 is so small as to make the heater 11 essentially inoperable.
The use of the voltage dropping resistor 11 has several disadvantages. First, the dropping resistor 11 adds cost to the circuit. Further, the resistor 11 is normally in the range of from 5-10 watts and may require a special mounting and/or a heat sink. In addition, during the manufacturing process, there is always the possibility that the dropping resistor could be omitted which may result in either an inoperative timer motor during the automatic drying cycle or worse, a burned out timer motor. Therefore, in an effort to continually seek to improve circuit performance, reliability and economy, there is a need to provide a dryer control circuit that does not require and have the disadvantages of a control circuit employing a voltage dropping resistor.